Our eyes capture an image by focusing light onto the retina using our lens. So, what the heck does that mean? An object becomes visible when light rays bounce off of it. Although, the reflected light gets scattered in all directions. If our eyes did not have lenses, our retinas wouldn't be able to capture images since it would just be a blurry mess of scattered light rays.
Our lens bends light rays by taking advantage of a property called refraction: When light passes through an object with a different refraction index, it changes directions. When our lens becomes more convex, the focus point will move closer toward our lens, and vice versa. This process is called accommodation.
People with myopia usually have longer eyeballs: where light is focused on a normal person's retina, it will be focused in front of a myopic person's retina. Meanwhile, people with hypermetropia have short eyeballs; Therefore, the focus point would be located behind their retinas. In older people, farsightedness is usually caused by the stiffening of the lens. This condition is called presbyopia. Contrary to previously mentioned conditions, astigmatism is caused by the cornea instead of the lens.